DEREFERENCING REFs:

Accessing  the  object  referred  to  through a  REF  is  known as  DEREFERENCING  the REF. Oracle gives the DEREF operator to do this.  The Dereferencing is a Dangling REF results in a null object.

REFs:

In the relational model, foreign keys express many-to-one relationship.   An Oracle with the objects option gives a more efficient means of expressing many-to-one relationships when the "one" side of the relationship is a row object.   Oracle provides every row object a unique, immutable identifier, known as an OBJECT IDENTIFIER. The REFs only describe a relationship in one direction.